This invention relates to a lash adjuster for automatically adjusting the valve clearance in a valve actuator of an internal combustion engine.
A valve actuator for opening and closing intake or exhaust valves (hereinafter simply “valves”) by rotating its cam includes a lash adjuster mounted between the cam and the valve to automatically adjust the valve clearance therebetween.
U.S. Pat. No. 4,548,168 discloses such a lash adjuster which includes a lifter body having an end plate kept in contact with a cam and formed with a blind threaded hole in its bottom surface. An adjuster screw is in threaded engagement with the threaded hole and is biased axially by an elastic member mounted between the top end of the screw and the closed end of the threaded hole. The female threads of the threaded hole and the male threads of the adjuster screw are serration-shaped so that the pressure flank to which a push-in load applied to the adjuster screw is applied has a larger flank angle than the clearance flanks.
This lash adjuster is mounted between a cam and the stem of a valve. A valve spring biases the valve toward the cam to press the end of the valve stem against the bottom end of the adjuster screw. As the cam rotates with the valve stem pressed against the adjuster screw, the valve stem is moved up and down between its open position and closed position of the valve.
If a valve clearance forms between the top end of the valve stem and the adjuster screw due e.g. to thermal expansion of the cylinder head, under the force of the elastic member, the adjuster screw will move axially downward while turning in one direction with the clearance flanks of the screw sliding along the clearance flanks of the nut until the valve clearance disappears.
Conversely, if the pressure from the valve stem is applied to the adjuster screw, the adjuster screw is pushed up or retracts until the axial play between the pressure flanks of the female threads and the male threads disappears. Once the play disappears, the adjuster screw cannot be pushed up any further because the frictional force between the pressure flanks is large.
But if, for example, the valve seat is worn, when the engine is started, large force will be momentarily applied to the adjuster screw from the stem when the stem rises and abuts the screw, thus pushing up the screw against the large frictional force between the pressure flanks, until the valve face is completely seated on the valve seat. Thus, it is possible to completely shut the valve when the base circle of the cam contacts the end plate of the lifter body, even if the valve seat is worn. This prevents pressure leakage. In this case, the adjuster screw is pushed up until the axial gaps between the pressure flanks disappear after the force from the valve stem has disappeared.
On the other hand, in a situation where no valve clearance adjustment is necessary while the engine is running, the adjuster screw scarcely turns and moves axially within the gap or play between the female threads of the threaded hole and the male threads of the adjuster screw.
That is, the pressure flanks of the male threads of the adjuster screw repeatedly collide against and move away from the pressure flanks of the female threads.
In the valve assembly, lubricating oil such as engine oil is present. Such lubricating oil inevitably flows into between the pressure flanks and forms an oil film. When the adjuster screw undergoes an axial load, the pressure flanks tend to expel such oil film when they move toward each other. The oil film produces a pressure against the pressure from the pressure flanks. The oil film has a load-bearing limit. If the pressure applied from the pressure flanks to the oil film exceeds this maximum load limit, the oil film will break up and will be discharged. The pressure flanks of the male and female threads thus directly contact each other. Since the friction between the pressure flanks is large, it prevents the adjuster screw from turning.
On the other hand, if the pressure applied to the oil film from the pressure flanks balances with the load-bearing limit of the oil film, the oil film will remain therebetween. That is, the pressure flanks are practically separated from each other by the oil film. Thus, the adjuster screw tends to retract toward the closed end of the threaded hole while turning due to reduced friction between the pressure flanks. This reduces the valve lift amount.
Generally it is known that the smaller the total area of the pressure flanks, the smaller the load-bearing limit of the oil film. Also, by dividing the pressure flanks to the greater number of sections for a given area of the pressure flanks, it is also possible to reduce the load-bearing limit of the oil film.
JP patent publication 03-501758 proposes a lash adjuster including an adjuster screw having a plurality of circumferential grooves formed in the pressure flanks to reduce the load-bearing limit of oil film present between the pressure flanks, so that the oil film can be expelled smoothly and quickly, thereby stabilizing the lift of the valve.
JP patent publication 2000-130114 discloses a lash adjuster in which a plurality of axial grooves are formed in the inner periphery of a threaded hole formed in the lifter body at circumferential intervals to circumferentially divide the pressure flank of the female thread into many small sections, thereby expelling the oil film smoothly and quickly.
The lash adjuster disclosed in either of the abovementioned Japanese publications has one problem that when the pressure flanks of the female and male threads are abraded and get worn, the contact surfaces tend to become smooth. This reduces friction between the pressure flanks to such an extent as not to be able to check the rotation of the adjuster screw.
One way to avoid this problem is to roughen the pressure flanks of the female and male threads. But ruggedness formed by such rough surfaces are not sufficient to efficiently discharge oil film. Especially in a low-temperature condition in which the viscosity of lubricating oil becomes high, it takes time to expel oil and thus the adjuster screw can slip and turn, so that the valve lift decreases.
The load-bearing limit of the oil film varies with the distance between the opposed pressure flanks, their area, shape and speed at which they move toward each other, viscosity of lubricating oil, etc. FIGS. 11A and 11B are graphs showing the relationship between the distance between the opposed pressure flanks and the ambient temperature and the load-bearing limit of the oil film.
The graph of FIG. 11A shows the results for a lash adjuster in which the female threads of the threaded hole formed in the lifter body and the male threads of the adjuster screw have pressure flanks and clearance flanks provided alternating with the pressure flanks so that the pressure flanks have a greater flank angle than the clearance flanks. This lash adjuster has no axial grooves as used in the lash adjuster disclosed in JP patent publication 2000-130114. The graph of FIG. 11B shows the results for the same lash adjuster as used in FIG. 11A except that it has the axial grooves as used in JP publication 2000-130114.
The graph of FIG. 11A shows that the load-bearing limit of the oil film increases sharply with increase in the viscosity of the oil film, which in turn increases with reduction in the temperature. Even while the distance between the pressure flanks of the female and male threads is relatively large, the pressure-bearing force of the oil film may balance with the axial load transmitted from the cam to the valve through the lash adjuster at low temperature.
FIG. 11B shows that the shorter the distance between the pressure flanks, the greater the load-bearing force of the oil film. Thus, if the pressure flanks become smooth due to wear, the distance therebetween before they contact decreases. Thus, even if the pressure flanks are circumferentially divided into small sections, the load-bearing force of the oil film can grow rather large.
An object of this invention is to provide a lash adjuster in which oil film disposed between the opposed pressure flanks can be expelled smoothly and quickly from when the distance between the opposed pressure flanks is large to the instant they contact, and even after the pressure flanks have been worn due to long use, friction sufficient to keep the adjuster screw from turning is maintained between the opposed pressure flanks, so that stable valve stroke is maintained.